Newton’s 3rd Law For every action…..

Newton's Third Law “For every action there is an equal and opposite reaction.” Newton’s third law discusses pairs of objects and the interactions between them. The normal force is an example we’ve already seen.

Newton’s Third Law Another (and probably better) way of saying this is: For every force, there is an equal but opposite reaction force. This means that when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted.

You might be wondering… If the wall is pushing on me, then why don’t I move?

Another Example: When you throw a ball, you exert a force on the ball. That part is obvious. It may not feel like the ball is exerting an equally strong force on you. But what if you were standing on a skateboard, and then you threw a ball as hard as you could? What would happen?

Action and Reaction Forces Don’t Cancel Newton’s Third Law Action and Reaction Forces Don’t Cancel The forces exerted by two objects on each other are often called an action-reaction force pair. Either force can be considered the action force or the reaction force. Action and reaction force pairs don’t cancel because they act on different objects.

Newton’s Third Law When you jump, you push down on the ground. The ground then pushes up on you. It is this upward force that pushes you into the air.

Newton’s Third Law When a bird flies, its wings push in a downward and a backward direction. This pushes air downward and backward. By Newton’s third law, the air pushes back on the bird in the opposite directions—upward and forward. This force keeps a bird in the air and propels it forward.

Newton’s Third Law When you walk forward, you push backward on the ground. Your shoe pushes Earth backward, and Earth pushes your shoe forward.

3rd Law Questions Identify the action reaction pair for a man paddling a boat. A. Man pushes on paddle, boat pushes on man. B. Paddle pushes on water, water pushes on paddle. C. Paddle pushes on water, water pushes on man. D. Man pushes on paddle, water pushes on boat.

3rd Law Questions A acts on B – B acts on A Choose the answer that only involves two objects and follows the idea of : A acts on B – B acts on A

Falling objects What force is being exerted that makes an object falls to the ground? What is the reaction force?

Think of a falling apple…. The earth exerts a downward force on the apple (gravity), so the apple exerts an equal upward force on the earth.

Large and Small Objects If the apple is exerting an upward force on the Earth, why doesn’t the earth move upward? Earth has so much mass compared to the apple that it does not move noticeably when the apple pulls on it.

The forces are the same… Force on apple = Force on earth However, aapple = aearth =

Newton's Third Law The astronauts on a space walk have a problem when they need to move around in space: There is nothing to push on. The solution is to throw something opposite the direction they want to move. This works because all forces always come in pairs.

Newton’s Third Law A Rocket Launch When the rocket fuel is ignited, a hot gas is produced. As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.

A Rocket Launch This downward push by the rocket on the gas molecules is the action force. The reaction force is the upward push on the rocket engine by the gas molecules. This is the thrust that propels the rocket upward.

Newton's Third Law Allows us to move!

Check your understanding…. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

Check your understanding… Many people are familiar with the fact that a rifle recoils when fired. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ... a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet.

Practice Problems If an 50 Kg man jumps off a 200 kg boat with force of 100 N, what is the acceleration of the boat? An 80 kg astronaut throws a 10 kg air tank with an acceleration of 3 m/s. What’s the astronaut’s acceleration?